Two principal types of rotary drill bits are employed for rock drilling for oil wells, recovering core samples, and the like. One type of rotary rock drill is a drag bit. Some of these have steel or hard faced teeth, but primarily they are set diamond drills such as described in U.S. Pat. No. 3,174,564. Typically in a set diamond drill the face is coated over much of its area with a hard material in which are embedded or "set" numerous diamonds. Diamonds are brazed into a wear resistant substrate. The diamonds protrude from the surface of the matrix and when the drill is used they rub on the rock, abrading shallow tracks and cutting primarily by a combination of compressive and shearing action.
Another type of bit uses rolling cone cutters mounted on the body of the drill bit so as to rotate as the drill bit is rotated. The use of rolling cone cutters in drilling rock is a well-known and long-established art. A typical rock bit includes three roller cutters, each having a generally conical configuration, and each occupying part of a separate 120.degree. sector above the face of the well bore. Some rock bits for special purposes have two, four or more cutters although three cone bits are probably most commonly used. Each roller is equipped with a number of generally circular rows of inserts or cutting elements. Some cones have hardened steel teeth integral with the cone. Many cones have tungsten carbide inserts or other hard material forming the cutting elements. As the roller rotates the work surfaces of the inserts of each row are applied sequentially in a circular path upon the face of the rock that is being drilled. As the rolling cone cutters roll on the bottom of the hole being drilled, the teeth or carbide inserts apply a high compressive load to the rock and fracture it. The cone axes can be "offset" a small amount from an intersection with the centerline of the rock bit to enhance lateral loading on the rock being drilled. The cutting action in rolling cone cutters is typically by a combination of crushing and chipping.
There are several distinct shapes of tungsten carbide inserts which are standard in the industry for rolling cone cutters, such as a conical, the double cone, the semi-projectile, and the chisel crest. All of these insert shapes, however, are generally characterized in that they comprise a cylindrical base for mounting in a rolling cone cutter and an end converging to a work surface. The work surfaces are blunt-pointed with a somewhat wedge-shaped configuration, meaning that the first engagement with the surface of the rock is but a relatively small surface area, but when indentation into the surface of the rock has progressed, the width or thickness of the cutting element which then comes into contact with the rock is greater.
Combinations of drag bits and rolling cone bits have been proposed. For example, U.S. Pat. No. 3,174,564 to E. A. Morlan for a "Combination Core Bit", has a cylindrical crown encrusted with set diamonds for cutting an annulus around a core. The set diamonds protrude from the matrix tiny distances in the conventional manner. A plurality of rolling cone cutters with carbide inserts are mounted in special recesses around the cylindrical crown for cutting an outer annulus of considerably greater area than the inner annulus cut by the diamonds. Also, U.S. Pat. No. 1,506,119 describes a combination rotary cutting/diamond bit.
In operation, a rolling cone drill bit is attached to the lower end of a drill string and rotated about the longitudinal axis of the drill bit on the bottom of a bore hole. Thus, the rolling cone cutters are caused to rotate, and as weight is applied to the bit by the weight of the drill string, the tungsten carbide inserts of the rollers crush, chip, gouge, and scrape the formation upon which the bit is rotated depending on the presence or absence of skew or "offset" of the cone axis. The particles of rock formation thus dislodged are carried out of the bore hole by drilling fluid which is pumped downwardly through the drill stem and bit head, returning to the surface of the earth via the space between the drill stem and the wall of the bore hole being drilled.
The tungsten carbide inserts along the periphery of a bit and which define the diameter of a hole being drilled are known as gage inserts. As the rolling cone cutters rotate, the gage inserts scrape against rock at the periphery of the hole being drilled as well as contact the bottom of the hole to dislodge rock formation by compression and gouging. Of all the working inserts of a rolling cone cutter, the gage inserts are most susceptible to wear because they undergo both abrasion and compression as they scrape against the periphery of a bore hole to dislodge rock. Any appreciable amount of wear on the gage inserts is undesirable because this could result in an undersized bore hole. If a bore hole is drilled undersized, then when a replacement drill bit is inserted toward the bottom of the bore hole, the replacement bit can pinch against the undersized portion of the hole and experience undue gage surface and bearing wear in reaming the undergage hole, thereby compounding the problem.
Rock bits are made with the gage diameter held within close tolerances. Thus, for example, with a rock bit having a nominal gage diameter of 7 and 7/8 inch, a standard specification calls for an actual gage diameter not less than 7 and 7/8 inch, and excess diameter no more than 1/16 of an inch. The gage diameter of the bit is the diameter across the outermost gage inserts on the cones. Thus, for example, the gage of a rock bit is determined by fitting a ring gage over the three cones. A 7 and 7/8 inch diameter bit would not fit into a ring gage having a diameter smaller than 7 and 7/8 inch but would fit into a ring gage having a diameter of 7 and 15/16 inch.
The cutter cones on a rock bit are also provided with flat faced tungsten carbide inserts on a portion of the cone which is intermittently adjacent the wall of the hole being drilled and which does not come in contact with the bottom of the hole. Such inserts are mounted substantially on the nominal gage of the rock bit. These inserts are known as the heel row. The abrasion resistant tungsten carbide inserts on the heel row help prevent the rock bit from going under gage. The heel inserts on the rock bit are subject to considerable abrasion and it is desirable to enhance their wear resistance.
Excessive wear on gage and heel inserts can occur even though gage inserts generally are made of tungsten carbide, either by itself or combined with other materials such as cobalt. Although tungsten carbide exhibits good compressive strength, it has relatively poor abrasion resistance when compared to diamond material. Therefore, the gage cutting elements tend to wear faster than other cutting elements, and thereby can be a limiting factor on the life of a drill bit. Excessive wear due to abrasion on the gage cutting elements necessitates premature replacement of the drill bit. Replacement is a time-consuming and expensive process, especially in deep bore holes, since the entire drill string must be removed from the hole in order to change the bit. Also, tungsten carbide inserts in all positions in a rolling cone cutter can exhibit poor wear resistance when drilling through formations interlaced with regions of hot wear containing corrosive salts such as when drilling for sources of geothermal energy.
Therefore, there is a need for tungsten carbide inserts for rolling cone cutters having high compressive strength, good resistance to abrasion, and good wear resistance when used to drill for sources of geothermal energy.